The reuse of historical penitentiary architecture is an indispensable measure to guarantee its preservation. This leads to answers that are often difficult to reconcile with the principles of conservation. In fact, although it is possible of referring to the exception, it is essential to confront the reasons invoked by European legislation, which often lead to putting technical-functional reasons before the values of history and memory. In this context there are also reasons of an economic and profit-making nature, the latter often facilitated by actions of spectacularisation of spaces. Moreover, this generates the compromise of their typological, formal, structural, distributive characteristics (specific to single buildings, also in relation to the dialogue they have with the city). They usually manage to resist this condition, thanks to their nature as monuments, especially if reinforced by seriality, regularity and symmetry. Starting from the assumption that, as Oswald Mathias Ungers states, every realisation must never "give up an idea", and in accordance with the thought on "critical modification" expressed by Vittorio Gregotti, the study interprets historical prisons according to a principle of continuity in time and space, and recognises in reuse the unavoidable interrelationships between past, present and future. The methodological path adopted for the definition of project scenarios for disused prisons in the Sardinian context included, as a starting point, the results that emerged from various disciplinary investigations from close or apparently distant fields. The project proposals, read from a comparative approach, highlight the need for three aspects: the relationship between Architecture and the City, the choice of appropriate functions and respect for the original architectural and typological aspects.